Current mixer (i.e., receiver) techniques are based on diode rings, field-effect transistor (FET) resistive switches, and differential pairs of transistors. These current mixer techniques use a local oscillator to switch the radio frequency (RF) on and off, and need the local oscillator to be larger than the RF to ensure that the local oscillator dominates the RF signal simultaneously present in the diode ring or FET resistive switch. The Fourier analysis of the RF signal switched on and off at the local oscillator rate yields an intermediate frequency (IF) as one of the components, thus giving mixing action. With multiple RF frequencies present, additional Fourier components may arise, more than just difference frequencies, and these additional components can be near in frequency to the IF. These additional components mask the desired IF signals and are generally referred to as spurious signals. Minimizing the effect of these spurs requires still more local oscillator power. What is desired is an ideal multiplier or mixer with the ability to reduce local oscillator drive requirements in radar systems so as to reduce the power consumption and the number of parts in the local oscillator chain.
The Gilbert multiplier circuit (also referred to as the Gilbert multiplier or Gilbert cell mixer) is mathematically an ideal multiplier. It is based on transistors with an exponential transfer characteristic in a cross-coupled differential pair configuration and on transistors with a linear transfer characteristic in the emitter current sources. The Gilbert multiplier is described, for example, in B. Gilbert, “A precise four quadrant multiplier with subnanosecond response,” IEEE J. Solid-State Circuits, December 1968, pp. 365-373 and B. Gilbert, “A new wide-band amplifier technique,” IEEE J. Solid-State Circuits, December 1968, pp. 352-365, which are incorporated herein by reference. However, technologies that have been available for the last several decades, such as bipolar junction transistors (BJTs), complementary metal oxide semiconductor (CMOS) transistors, heterojunction bipolar transistors (HBTs), high electron mobility transistors (HEMTs), and silicon germanium (SiGe) transistors, do not satisfy both operating regions required by the Gilbert multiplier circuit. For example, BJTs, HBTs, and SiGe transistors satisfy the exponential control characteristic, but they do not satisfy the linear control characteristic. Tricks are used to try to achieve a linear control characteristic from BJTs, HBTs, and SiGe transistors but in the end these tricks compromise linearity and do not achieve exceptionally low inter-modulation products. CMOS transistors and HEMTs do not have either an exponential or a linear control characteristic in pure form and do not achieve exceptionally low inter-modulation products in the Gilbert circuit.